In a scanning electron microscope (“SEM”), a primary beam of electrons is scanned upon a region of a sample that is to be investigated. The energy released in the impact of the electrons with the sample causes the liberation of other charged particles in the sample. The quantity and energy of these secondary particles provide information on the nature, structure and composition of the sample. The term “sample” is traditionally used to indicate any work piece being processed or observed in a charged particle beam system and the term as used herein includes any work piece and is not limited to a sample that is being used as a representative of a larger population. The term “secondary electrons” as used herein includes backscattered primary electrons, as well as electrons originating from the sample. To detect secondary electrons, a SEM is often provided with one or more secondary electron detectors.
In a conventional SEM, the sample is maintained in a high vacuum to prevent scattering of the primary electron beam by gas molecules and to permit collection of the secondary electrons. However, wet samples such as biological samples are not suitable for observation in a high vacuum. Such samples experience evaporation of their fluid content in the vacuum before an accurate image can be obtained, and the evaporated gas interferes with the primary electron beam. Objects that outgas, that is, solids that lose gas at high vacuum, also require special consideration.
Electron microscopes that operate with the sample under a relatively high pressure are described, for example, in U.S. Pat. No. 4,785,182 to Mancuso et al., entitled “Secondary Electron Detector for Use in a Gaseous Atmosphere.” Such devices are better known as High Pressure Scanning Electron Microscopes (HPSEM) or Environmental Scanning Electron Microscopes. An example is the Quanta 600 ESEM® high pressure SEM from FEI Company.
In an HPSEM, the sample that is to be investigated is placed in an atmosphere of a gas having a pressure typically between 0.1 Torr (0.13 mbar) and 50 Torr (65 mbar), and more typically between 1 Torr (1.3 mbar) and 10 Torr (13 mbar), whereas in a conventional SEM the sample is located in vacuum of substantially lower pressure, typically less than 10−5 Torr (1.3×10−5 mbar). The advantage of an HPSEM as compared to a conventional SEM is that the HPSEM offers the possibility of forming electron-optical images of moist samples, such as biological samples, and other samples that, under the high vacuum conditions in a conventional SEM, would be difficult to image. An HPSEM provides the possibility of maintaining the sample in its natural state; the sample is not subjected to the disadvantageous requirements of drying, freezing or vacuum coating, which are normally necessary in studies using conventional SEMs and which can alter the sample. Another advantage of an HPSEM is that the ionized imaging gas facilitates neutralization of electrical charges that tend to build up in insulating samples, such as plastics, ceramics or glass.
In an HPSEM, secondary electrons are typically detected using a process known as “gas ionization cascade amplification” or “gas cascade amplification,” in which the secondary charged particles are accelerated by an electric field and collide with gas molecules in an imaging gas to create additional charged particles, which in turn collide with other gas molecules to produce still additional charged particles. This cascade continues until a greatly increased number of charged particles are detected as an electrical current at a detector electrode. In some embodiments, each secondary electron from the sample surface generates, for example, more than 20, more than 100, or more than 1,000 additional electrons, depending upon the gas pressure and the electrode configuration.
An HPSEM limits the region of high gas pressure to a sample chamber by using a pressure-limiting aperture (PLA) to maintain a high vacuum in the focusing column. Gas molecules scatter the primary electron beam, and so the pressure limiting aperture is positioned to minimize the distance that the primary electron beam travels in the high pressure region in order to reduce interference with the primary beam, while providing a sufficient travel distance between the sample and the detector for adequate gas cascade amplification of the secondary electron signal.
An HPSEM as described in U.S. Pat. No. 4,785,182 comprises a vacuum envelope having a pressure limiting aperture, an electron beam source located within the vacuum envelope and capable of emitting electrons, one or more focusing lenses located within the vacuum envelope and capable of directing an electron beam emitted by the electron source through the pressure limiting aperture, beam deflectors located within the vacuum envelope and capable of scanning the electron beam, and a sample chamber including a sample platform disposed outside the high vacuum envelope and capable of maintaining a sample enveloped in a gas at a desired pressure.
While an HPSEM can observe moist biological samples, problems still exist with such observations. For example, when hydrated materials are observed at room or body temperature, water tends to condense on all surfaces within the sample chamber. Such condensation can interfere with the operation of HPSEM, as well as cause corrosion and contamination.
Charged particle beams, such as electron beams or ion beams, can also be used to induce a chemical reaction to etch a sample or to deposit material onto a sample. Such processes are described, for example, in U.S. Pat. No. 6,753,538 to Musil et al. for “Electron Beam Processing.” The process of a charged particle beam interacting with a process gas in the presence of a substrate to produce a chemical reaction is referred to as “beam chemistry.” The term “processing” as used herein includes both processing that alters the sample surface, such as etching and deposition, as well as imaging. The term “processing gas” is used to include a gas that is used for imaging or a gas that is used together with the charged particle beam to alter the sample. The term “imaging gas” is used to include a gas that is used primarily for imaging. The classes of gasses are not mutually exclusive, and some gasses may be used for both altering the sample and for forming an image. For example, water vapor can be used to etch a sample that includes carbon. Water vapor can also be used to form an image of samples that include other materials.
Conventional HPSEMs are not well adapted for efficient beam chemistry. One problem with using a HPSEM system for beam chemistry is the presence of impurities such as H2O inside the sample chamber. Beam chemistry typically involves a three step procedure that precedes processing. First, the sample chamber is pumped to high vacuum whereby the chamber pressure is lower than 10−5 mbar using a high vacuum pump such as a turbomolecular pump backed by a roughing pump such as a scroll pump. Second, the sample chamber is isolated from the high vacuum pump, and third, the chamber is filled with a precursor gas used for beam chemistry. For example, WF6 and Pt(PF3)4 are used for electron beam-induced deposition of materials that contain W and Pt, respectively, and XeF2 is used to etch materials such as SiO2 and Cr. A problem with this approach is that the second and third steps and subsequent beam chemistry process steps cause an increase in the concentration of gaseous impurities such as H2O inside the sample chamber up to a pressure well above the initial base pressure of 10−5 mbar. This increase in impurity concentration is the result of a dramatic reduction in the pumping rate of the chamber caused by the above step 2. The impurities are introduced by desorption from surfaces inside the sample chamber, and diffusion through o-rings typically used on SEM and HPSEM chambers. In a typical chamber, the pressure of background impurities will increase from the high vacuum base pressure of less than 10−5 mbar to a HPSEM base pressure greater than 10−3 mbar. The exact HPSEM base pressure is a function of the inner surface area of the chamber, the number and size of o-rings used to isolate the chamber from laboratory atmosphere, and the size of the pressure limiting aperture that restricts the pumping speed of the HPSEM sample chamber through the differentially pumped electron optical column. The impurities are comprised primarily of H2O, N2 and O2 and do not interfere with historically conventional HPSEM operation which entails filling the chamber with a gas such as H2O vapor for the purpose of charge control and stabilization of vacuum-incompatible samples. The impurities do, however, interfere with beam chemistry because molecules such as H2O and O2 react with and cause the decomposition of deposition and etch precursors such as most organometallics, WF6, MoF6, Pt(PF3)4, XeF2, F2 and Cl2. The impurities also occupy surface sites at the sample surface thereby reducing the adsorption rate of precursor molecules used for beam chemistry; and cause the oxidation of materials such as W and Mo during deposition and thereby alter the composition and functional properties of the deposited material. The HPSEM base pressure can be decreased by increasing the size of the pressure limiting aperture between the sample chamber and the electron optical column. This is, however, undesirable as it increases the gas flow rate into the column and decreases the maximum usable sample chamber pressure. The HPSEM base pressure can also be decreased by pumping the sample chamber using a roughing pump such as a scroll pump. This, however, is also undesirable as it increases the consumption and exhaust rates of precursor gases, which are often expensive and toxic, and reduces the lifetime of the roughing pump. The HPSEM base pressure can also be decreased using techniques employed in ultra-high vacuum technologies, such as the use of knife-edge metal-to-metal vacuum seals in place of o-rings, and baking of the sample chamber prior to analysis. These approaches are, however, incompatible with the cost and flexibility expected of commercial variable pressure SEM (VPSEM) systems.
Another problem with using a HPSEM system for beam chemistry is the considerable time required to introduce and evacuate gases from the sample chamber. The sample chamber in a conventional HPSEM includes a gas inlet through which a gas is introduced by way of a leak valve. The gas then migrates throughout the sample chamber. Some of the gas molecules escape through the PLA into the column, where they are removed by a vacuum pump that maintains the column at a low pressure. The inlet leak valve is adjusted so that a desired equilibrium pressure is achieved, with the gas escaping through the PLA into the column just matching the gas introduced through the leak valve. The HPSEM typically begins processing a sample only after equilibrium is achieved. It takes a considerable amount of time for the gas to reach an equilibrium pressure, particularly if the vapor pressure of the beam chemistry precursor is similar to the desired operating pressure of the sample chamber. For a large sample chamber volume of 30 liters, such as that used in a common HPSEM and dual beam systems, it can take up to 30 minutes for the partial pressure of the process gas to reach equilibrium. A “dual beam” system employs two beams, such as an electron beam and an ion beam, implemented on a single sample chamber.
The above problem is compounded when a process entails injecting multiple process gases into the sample chamber. Typically, there is a pressure gauge downstream of the needle valve on the sample chamber side. The pressure gauge typically measures the total pressure in the sample chamber and is incapable of separately measuring the partial pressures of multiple process gases in a mixture. Thus, it is difficult to know when the desired partial pressure of each of the gases has been achieved.
When performing beam chemistry processing in a conventional SEM, focused ion beam (FIB), or dual beam system, the system operator will typically obtain a charged particle beam image of the sample to navigate to an area that is to be processed by etching or depositing material. After performing the beam processing operation, the operator will typically obtain another charged particle beam image of the sample to evaluate the results of the process. Because different gases are typically used to process and image in an HPSEM, the sequence of image, process, and image would require multiple changes of the gas in the chamber. If some process gas remains in the chamber during imaging, the sample may be unintentionally modified by the beam during the imaging operation. Because of the considerable time required to fully evacuate one gas and then to reach equilibrium pressure with another gas, such multiple step operations are not practical in an HPSEM. The processing time is further increased in some cases because the molecules of some gases used in beam chemistry tend to have very long residence times on the chamber walls, and take longer to fully evaporate and to be removed from the sample chamber.
Another reason why HPSEMs are not generally used for beam chemistry is that corrosive process gases can degrade the HPSEM components. For example, certain process gases associated with beam chemistry can react spontaneously with components such as plastic tubing and are very dangerous to human health. Gases like XeF2 and MoF6 can make plastic gas tubing brittle and eventually cause leaks of dangerous gases into the surrounding environment.
The gaseous environment of a HPSEM sample chamber enables real-time SEM studies of dynamic processes in which a sample is modified through thermal annealing in the presence of a reactive gas. For example, a sample such as iron or steel can be oxidized by heating the sample to a high temperature (e.g., 850° C.) in an O2 environment, causing microstructural changes that can be imaged in real time during oxidation. However, the range of studies that can be performed in a HPSEM is limited by the abovementioned impurities, such as H2O and O2, which are responsible for the VPSEM base pressure. These impurities inhibit sample modification processes like heat-induced reduction caused by a reducing gas like H2 and many forms of chemical vapor deposition (CVD) due to unintended reactions between the impurities and the sample, or the impurities and the process precursor molecules.
PCT/US2008/053223, which is assigned to the assignees of the present application and which is hereby incorporated by reference, describes several configurations of environmental cells that allow HPSEM operation while solving some of the problems described above. By “environmental cell” is meant an enclosure for providing an environment around the sample, typically a different environment than that present in a sample chamber in which the environmental cell is located. An environmental cell can solve some of the above problems by enhancing control of the sample environment, reducing the concentration of gaseous impurities present during HPSEM processing, and reducing the volume and inner surface area of the HPSEM process chamber. Embodiments of the present invention include design improvements to the environmental cell methodology to reduce cost and engineering complexity, and expand the usefulness of HPSEM for the investigation and application of beam chemistry and heat-induced gas-mediated processes such as oxidation, reduction and CVD. The improvements include improved control of the sample environment, facilitation of complementary “correlative” sample analysis techniques for studies and application of HPSEM beam chemistry and heat-induced gas-mediated processes, and a reduction in the number of materials that must be employed inside an environmental cell and consequent chemical incompatibilities between the materials and process precursors.